This invention relates to a screening finger tine assembly which comprises a holder, and a set of finger tines laterally spaced apart from each other in a row and each mounted in cantilever manner in a respective mounting in the holder.
Finger tine assemblies of this type may be mounted in a screen box, and typically each assembly has a bank of fingers arranged in a cassette, and the cassettes are arranged successively from an upper bulk material receiving end of the box to a lower discharge end of the box from which oversized material can be discharged. The banks of fingers in the cassettes can be arranged with varying spacing apart between the fingers from one cassette to another, to allow different sizes of screened material to pass downwardly between the fingers into the screen box, for collection and subsequent discharge to respective stockpiles corresponding to each required graded size range of screened material.
The screen box will usually have a vibrating mechanism coupled with it to apply vibration to the cassettes so that the fingers will vibrate and thereby promote the screening action and possibly assist in breaking down the bulk material into smaller fragments.
It is present practice to mount the inboard end of each finger tine in a steel bush which is welded or otherwise secured to a common hollow transverse carrier or mounting beam provided for the finger tines of each set (cassette).
The inboard end of the tine is usually tapered, in the sense that it reduces in diameter in a direction from the outboard end to the inboard end, and therefore the steel bush (which acts as a holder to provide cantilever mounting of the finger tine) has a matching internal tapered shape also, so that firm holding contact can be maintained with the entire outer surface of the tapered end of the finger tine.
The inboard end of the tine is usually clamped firmly within the bush by applying an axial force to the tine so as to press the tapered end of the tine firmly into engagement with the internal surface of the bush. This is achieved by tightening a clamping nut which reacts against an end face of the bush to apply necessary tensile force to the tine.
The cantilever mounting of each finger tine is therefore a very rigid assembly, and which must be robustly constructed in order to withstand the impact loadings to which the tines are subjected to. In this respect, it should be bourne in mind that bulk material to be screened e.g. site rubble, soil, stones etc. is usually dumped onto the receiving end of the screen box via an excavator bucket or dumper truck, and such material falls under gravity onto the fingers. Also, the fingers are caused to vibrate, in order to screen the material, and therefore substantial bending and shear loads are applied to the fingers, all of which have to be bourne by the mounting of the bushes in the hollow transverse carrier beam.
Therefore, during usage over a period of time, the shock and vibration loadings tend to cause cracks to be generated in the beam, and the bushes then are no longer held rigidly in position, and/or the tines also may snap at positions along their length. A failure of even a single one of the finger tines of a set or cassette has an adverse effect on the overall screening efficiency of the cassette, and necessary repairs of even a single tine represent substantial downtime in the operation of the screen box, which reduces overall operating efficiency.
The present invention has therefore been developed primarily with a view to address this problem and to provide, in simple manner, a cantilever mounting assembly for each finger tine which is better able to withstand shock and vibration loadings to which the tine is exposed during normal usage.